The present invention relates to a method and apparatus for connecting a well on the ocean floor with a wellhead "Christmas" tree, (i.e., the flow control valves) on a fixed or relatively fixed platform, such as a floating tension leg platform, or the like. More particularly, the present invention relates to a riser top joint used in completing such a connection that makes it unnecessary to precisely measure the distance between the well and the wellhead tree.
One of the benefits of a tension leg platform over other floating systems is the very small vertical oscillation that occurs. This enables the wellhead trees to be mounted within a few feet of a platform deck without the need for some complex form of motion compensation system. However, the use of a rigid riser system requires a precise measurement between the well on the ocean floor and the deck of the platform, in order to obtain a riser of the necessary length. Such precise measurement becomes increasingly difficult as the water depth moves from hundreds to thousands of feet deep.
It is an object of the present invention to make such an accurate measurement unnecessary. By providing a riser top joint that affords continuous or stepwise adjustability, the requirement of precision measurement between the well and the tree is obviated. The riser top joint of the present invention comprises a generally cylindrical pipe having a first internal diameter and a first outer diameter. A series of equally spaced generally annular protrusions extend outwardly from said first outer diameter to a second outer diameter providing a series of connection points. The protrusions extending above the upper surface of the deck of the platform comprise a first series of connection points for the wellhead tree that may be secured thereto by means of either a unitary or a split segmented collar. The protrusions extending below the lower surface of the deck comprise a second series of connection points for a riser tensioner to maintain essentially uniform tension on the riser despite the small vertical motion of the platform resulting from the wave-induced pendulum-like motion of the platform. The generally annular protrusions may most preferably take the form of a continuous spiral groove on the external surface of the riser permitting continuous adjustability.
The method of using the variable riser top joint in accordance with the present invention involves making up the riser string with the top joint of the invention, positioning the top joint such that the generally cylindrical protrusions extend both above the top surface and below the bottom surface of the well deck, cutting off any excess riser joint, securing a collar about said top joint at a point spaced from the top end thereof, attaching a wellhead tree to the top of said riser joint and packing off said wellhead tree.
Various other features, advantages and characteristics of the present invention will become apparent after a reading of the following specifications.